TIDA-00367: SMPS testing

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Replies: 15

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Part Number: TIDA-00367

Hi,

Following the thread

Have successfully built the SMPS and it appears to work in NO LOAD situations and with a 55W load too (24V/55W lamp load). The output voltage is about 19.6V. The SMPS appears okay with Short Circuit protection too.

Things went well, thanks to the folks who have chimed in on various aspects.

The issues that I've been seeing:

#1. There's a small noise. The noise varies with the load at the output. Initially, I thought it is from the transformer, that doesn't seem to be the case. The transformer has been manufactured by a professional vendor. It has been varnished, baked and so on.

Where else can the noise come from ? Looking around, I see people talking about capacitors making the noise, Is it really true ?

#2. Changed the load from the lamp to a Brushless DC motor and it's controller. The setup works with a commercial off the shelf SMPS quite okay. But when I do power from the SMPS under consideration, the setup goes really crazy. It appears that the controller gets RESET due to noise from the power supply.

The funnier part, is that the same motor and controller ran well from the SMPS under consideration for about a week. It was all of a sudden, I had a failure of a MLCC, Gate driver (DRV8305) and a MOSFET pair with the BLDC controller. After that the issue was identified and the motor runs perfectly with everything else, except with this SMPS that we are talking about.

As a result for testing purposes, added a 10,000uF/50V electrolytic capacitor along with a 470uH toroid inductor as a LC filter to filter out the power supply ripple. Still, with the LC filter, the motor and it's controller behaves exactly the same.

Tried looking at the ripple at the SMPS output with the load, the ripple voltage is less than 0.5V, looking at the waveform on a scope connected to the power rails.

Now, my query is, how to identify the issue with the SMPS ?

Thanks,

Manu

15 Replies

  • Update:

    The following tests conducted with a 12V automotive halogen lamp load

    @60W V=0V (SMPS shuts down, the sound of a cricket chirping at an interval of about 2.5s )

    @55W V=19.82V (The same sound as in the case of a NO LOAD, but it is at a slightly reduced level)

    @21W V=20.05V (the same sound as in the case of NO Load)

    @ 5W V = 20.14V (the previous sound exists, in addition a kind of braking noise which is slightly more prominent)

    NO LOAD V= 21.7V (the sound of a square wave, similar to a mosquito buzzing)

  • In reply to Manu Abraham:

    Update:

    Tried capturing the MOSFET gate waveform with an oscilloscope: Please find attached images.

    Funny to say, the weirdness as the scope probe was connected between the MOSFET Gate and Ground, the weirdness vanished. (That said, there is still a RESET happening but that's in the order of a few minutes.)

    Could this probably be a decoupling issue ?

    Thanks,

    Manu

  • In reply to Manu Abraham:

    Update:

    Following the tip with situation improving when the scope probe was introduced at the FET Gate, tried looking at the electrolytic capacitors, which looked okay, but nevertheless the electrolytic caps and the decoupling capacitors were replaced.

    There was no visible change at all, with those changes.

    The thought switched to common mode noise:

    Looking at the 1000pF capacitor connected in between the primary GND and the Output GND, the capacitor seemed to be fine, measuring the capacitance.

    Adding another 1000pF in parallel to it, reduced the issue very much, but the issue still does not go away completely.

    What else can I look at, which contributes to common mode noise, if that's the culprit ? Is there a possibility of transformer degradation, worsening the Common mode noise situation ? Any thoughts, someone ?

    No one's reading this thread ? Any suggestions ?

    Thanks,

    Manu

  • In reply to Manu Abraham:

    Feeling a bit silly replying to my own post, but hoping that someone would reply:

    #1. Measured between C2 GND and T1 GND Pin 11. The spikes are recurring, I guess these spikes are the one RESET ing the C2000 controller eventually. The voltage is between 2 GND ends!. It does look a bit strange, doesn't it ?

    #2.  The voltage waveform at the output of T1 Pin 8

    #3. The MOSFET Drain - PRI GND waveform

    #4. The FET Drain waveform with the LOAD connected and BLDC motor running.

    #5. The same as #1, but after removing C1(1000pf) across the main rectifier D2, with the LOAd connected and running. The noise amplitude seems to have reduced in comparison to #1, but there appears to be more noise ?

    Can someone please have a look ?

    Thanks,

    Manu

  • In reply to Manu Abraham:

    Feeling a bit silly replying to my own post, but hoping that someone would reply:

    #1. Measured between C2 GND and T1 GND Pin 11. The spikes are recurring, I guess these spikes are the one RESET ing the C2000 controller eventually. The voltage is between 2 GND ends!. It does look a bit strange, doesn't it ?

    #2.  The voltage waveform at the output of T1 Pin 8

    #3. The MOSFET Drain - PRI GND waveform

    #4. The FET Drain waveform with the LOAD connected and BLDC motor running.

    #5. The same as #1, but after removing C1(1000pf) across the main rectifier D2, with the LOAd connected and running. The noise amplitude seems to have reduced in comparison to #1, but there appears to be more noise ?

    Can someone please have a look ?

    Thanks,

    Manu

  • In reply to Manu Abraham:

    Update:

    The problem mostly fixed by adding a Common Mode Choke at the SMPS output. Attached picture. So, the culprit was indeed Common Mode noise as identified in the previous post. That said, even though the problem has nearly been fixed, once in a while, some noise does really pass through.

    Any suggestions to to filter it out ?

    Maybe increase the number of turns, or maybe even use copper wires instead ? Any suggestions ?

    Thanks,

    Manu

  • In reply to Manu Abraham:

    Some more update:

    I was able to get rid of the noise as well as the audible noise when I introduced a Load of ~0.55A.

    Dummy Load schematic

    With this load in parallel to the BLDC controller, all the odd behaviour with the BLDC setup vanishes away for good.

    Looking at the UCC28630 datasheet, it appears that the audible noise as well the weird behaviour occurs most likely

    when the UCC28630 is running between the P0-P1 region. Also, when the SMPS is operating in that region, there is a sound of a mustard seeds bursting open in hot oil. It is with that noise, the BLDC controller goes berserk and reset's itself.

    The various modes appear to be controlled by Vcs, which is the the same as CS Pin Voltage ?

    Can someone clarify, please ?

    To increase Vcs can I simply increase the CS resistor value and expect that the UCC28630 moves operation away from P0-P1 region to P1-P2 and avoid the dummy Load altogether ?

    Any thoughts ? Anyone reading this thread ?

    Thanks,

    Manu

  • In reply to Manu Abraham:

    Hi Manu,

    I will contact designer to help answer this question, thanks.

    BR

    Jason

  • In reply to Yuan Tao:

    Hi Jason,

    Much appreciated, awaiting feedback on the subject.

    Thanks,

    Manu

  • In reply to Manu Abraham:

    Hi Manu,

    I have forward your questions to Designer S, Ramkumar (ramkumar.s@ti.com), or you can contact him by email directly, many thanks.

    BR

    Jason